Most batteries lose water, but if kept upright, won't lose acid. Some newer batteries are sealed and you can't add anything. Most of them have caps and you can add water. Often there is sort of a split ring indicating where to fill to. Adding water to batteries with caps is standard routine maintaince. It should extend the life of a battery. I would use a low mineral water, distilled or deionized. Some bottled drinking water may be OK.

If it's a serviceable battery, the ONLY thing to add is water, and it's done by removing the snap-on or screw-on service caps. If it's a sealed battery, replace it, and test the charging voltage to ensure the new battery doesn't get ruined in a similar manner.

First of all, why do you think the battery has a low electrolyte (acid) level? Did you physically look at it to verify that it is low on electrolyte or is the battery just low on capacity? If the battery is just low on capacity it's not necessarily low on electrolyte.

A car battery has six cells in it (each cell is roughly 2 volts, and the six cells therefore add up to 12 volts). Each cell has two plates in it and a liquid electrolyte between the plates. One plate is lead and the other is lead oxide. The electrolyte is a mixture of sulfuric acid and water. As the battery discharges, both plates turn into lead sulfate and the sulfuric acid turns into water. When you recharge the battery, the plates again turn into lead and lead oxide and some of the water turns back into sulfuric acid.

About the only way to lose the sulfuric acid is to have it physically spill out of the battery. A battery will much more easily lose water though. If you have a lot of current going through the battery, some of the water is going to electrolyze into hydrogen and oxygen, which will bubble up out of the electrolyte and can escape through the battery fill covers. This type of water loss is perfectly normal, and the solution is to simply add more water through the fill caps.

If you've spilled out a bunch of the electrolyte, then adding water will help, but you will probably need to have some sulfuric acid added back into it to get the balance of water and sulfuric acid back to the proper level. If all you do is add water then the battery may be a bit weak. Don't try adding sulfuric acid yourself unless you happen to own a hydrometer to check the acid level (and you know how to use it).

While some water loss is normal, overcharging the battery will cause it to electrolyze the water into hydrogen and oxygen much faster, which will result in excessive water loss from the battery. This is why Gary T said to have the charging voltage tested. Having the electrolyte level too low for too long will ruin the plates inside the battery.

If it's a sealed battery, just replace it. If the battery has lost capacity and isn't low on electrolyte, replace it. If you spilled some of the electrolyte you'll want to replace it with the proper mix of water and sulfuric acid, but water will do until you can take it into a shop that has sulfuric acid available. If it's low on electrolyte just due to normal use, then just add water. If it has been low on electrolyte for a very long time, the plates may be damaged and you may be better off just replacing the battery.